Increasingly dire warnings of the dangers of climate change by the world's scientific community combined with greater public awareness and concern over the issue has prompted increased momentum towards global regulation aimed at reducing man-made greenhouse gas (GHGs) emissions, most notably carbon dioxide. Ultimately, a significant cut in North American and global CO2 emissions will require reductions from the electricity production sector, the single largest source of CO2 worldwide. According to the International Energy Agency's (IEA) GHG Program, as of 2006 there were nearly 5,000 fossil fuel power plants worldwide generating nearly 11 billion tons of CO2, representing nearly 40% of total global anthropogenic CO2 emissions. Of these emissions from the power generation sector, 61% were from coal fired plants. Although the long-term agenda advocated by governments is replacement of fossil fuel generation by renewables, growing energy demand, combined with the enormous dependence on fossil generation in the near to medium term dictates that this fossil base remain operational. Thus, to implement an effective GHG reduction system will require that the CO2 emissions generated by this sector be mitigated, with carbon capture and storage (CCS) providing one of the best known solutions.
The CCS process removes CO2 from a CO2 containing flue gas, enables production of a highly concentrated CO2 gas stream which is compressed and transported to a sequestration site. This site may be a depleted oil field or a saline aquifer. Sequestration in ocean and mineral carbonation are two alternate ways to sequester that are in the research phase. Captured CO2 can also be used for enhanced oil recovery.
Current technologies for CO2 capture are based primarily on the use of amine solutions which are circulated through two main distinct units; an absorption tower coupled to a desorption (or stripping) tower.
Biocatalysts have been used for CO2 absorption applications. For example, CO2 transformation may be catalyzed by the enzyme carbonic anhydrase as follows;
            CO      2        +                  H        2            ⁢      O        ⁢      ↔          carbonic      ⁢                          ⁢      anhydrase        ⁢            H      +        +          HCO      3      -      
Under optimum conditions, the catalyzed turnover rate of this reaction may reach 1×106 molecules/second.
There are some known ways of providing carbonic anhydrase in CO2 capture reactors. One way is by immobilising the enzyme on a solid packing material in a packed tower reactor. Another way is by providing the enzyme in a soluble state in a solution within or flowing through a reactor. Both of these methods provide benefits but also some limitations. Enzyme immobilized on a solid packing material limits the enzyme benefit since it has a limited presence in the thin reactive liquid film at the gas-liquid interface which has a thickness of about 10 μm; enzyme on packing is several millimeters from the gas-liquid interface. Soluble enzyme brings the optimal enzyme impact, however it cannot be easily separated from the solution and if the enzyme is not robust to intense conditions such as those used in desorption operations, it will be denatured and the process will require high levels of continuous enzyme replacement.
There is a need for a technology that overcomes some of these problems and challenges of the known techniques for providing biocatalysts such as carbonic anhydrase in CO2 capture reactors.